1. Field of the Invention
This invention relates to a method and an apparatus for suppressing noise in a radio receiver, and more particularly to suppressing noise due to electromagnetic interference generated by an ignition system that creates an electric spark to ignite a fuel/air mixture.
2. Description of the Related Art
The operation of radio receivers near ignition systems which create an electric spark to ignite a fuel/air mixture has always proved difficult because of the electromagnetic interference generated by such ignition systems. These ignition systems are commonly found in gasoline powered internal combustion engines used in automobiles, aircraft, marine vehicles, and other transportation media. The electric spark used in these engines, as well as the current flowing in the cables from the power source to the spark plug, generate electrical interference which is picked up by the vehicle's radio receiving antenna.
The interference picked up by the radio receiver from the ignition system is commonly known as impulse noise and causes a "popping" sound from the loudspeaker. In instances of lesser interference, this popping sound is merely annoying to the listener, but larger interference can render the radio unusable.
The traditional methods of controlling impulse noise have attempted to either prevent the coupling of the electrical interference with the receiving antenna or use a blanking circuit to prevent the flow of radio signals to the audio amplifier when the detected impulse noise reaches a predetermined level.
Attempts to decouple the electrical interference with the receiving antenna have included, among other methods, shielding the antenna from the interference source, grounding the engine to the vehicle body or frame, and locating the antenna at a location having reduced likelihood for electromagnetic interference pick up. These techniques merely reduce, but do not eliminate the pick up of interference by the antenna. Unfortunately, modern engines use very high levels of spark energy to improve operating efficiency and the above noted methods are not very effective in suppressing the pick up of impulse noise by the antenna.
On the other hand, noise blanking systems employ a blanking circuit disposed between a signal processing unit and a signal amplifying unit to prevent the impulse noise from reaching the amplifying stage when the detected impulse noise reaches a predetermined level. The noise blanking systems used today selectively activate the blanking function based on the detected presence of interference in the incoming signal. When interference of sufficient magnitude is detected, the blanking system sets a "flag" which activates the blanking function.
AM receivers use a single flag, commonly known as an automatic gain control (AGC) derived flag, which is derived from a sudden increase in signal level. FM receivers use two flags, an AGC derived flag like the AM receivers plus a high frequency audio information flag, which is created when a high frequency noise "pop" is detected. In a FM receiver, either or both flags may be used to control the noise blanking circuit.
However, the noise blanking systems used today have several shortcomings. First, these noise blanking systems are only effective for noise above a given threshold level. Incoming noise which is slightly below the threshold level will still reach the signal amplifying stage and may be heard on the loudspeaker. This is especially a problem when the desired signal level is low or close to the noise threshold level. Second, in situations where the desired signal level is moderate, the AGC circuitry will be desensitized by the moderate signal and no flags may be generated. In such a situation, "popping" noises will easily be heard from the loudspeaker. Limitations of AGC systems are commonly known and described in the literature. Third, since the levels of a series of impulses created while the engine is running will not all be identical, certain pulses may "escape" and be heard on the speaker because the pulse level was too low to create a flag. Further, on FM systems, certain lower energy pulses will not create enough high frequency energy to be detected by the secondary loop and thus escape.
Thus, conventional noise suppression systems are not totally effective in eliminating all impulse noises caused by electromagnetic interference generated by a vehicle's engine ignition system.